Aluminum species forming in Lewis acidic chloroaluminate and bis(trifluoromethylsulfonyl)amide (TFSA-) mixed ionic liquids having 1-butyl-3-methylimidazolium (BMI+) was investigated by potentiometry and Raman spectroscopy. A decrease in the concentration of [Al2Cl7]- and an increase in that of [AlCl4]- with addition of BMITFSA into acidic BMICl-AlCl3 were observed by Raman spectroscopy. The redox potential of Al(III)/Al in BMICl-AlCl3 (1:2 in molar ratio) was found to be -0.68V vs. an Ag|Ag(I) reference electrode, which was corresponding to -0.25V vs. the ferrocene/ferrocenium couple at 25°C. The potentiometry of an Al electrode in the mixed ionic liquids with different compositions using the Ag|Ag(I) reference electrode suggested formation of a mixed ligand aluminum complex, Al(TFSA)Cl2, indicating the instability of [Al2Cl7]- against TFSA-.